1. Field of the Invention
The present invention relates to an implantable heart stimulator, and a method for detecting a state of mechanical dyssynchrony of the heart of a patient, according to the preambles of the independent claims.
2. Description of the Prior Art
Many patients with advanced systolic heart failure exhibit significant intra- or interventricular conduction delays (IVCD) that disturb the synchronous beating of the ventricles so that they pump less efficiently. This delayed ventricular activation and contraction is referred to as ventricular dyssynchrony and is often seen as a wide QRS complex with a left bundle branch block morphology on ECG.
Ventricular dyssynchrony has been shown to have a number of deleterious effects on cardiac function, including reduced diastolic filling time, weakened contractility, protracted mitral regurgitation, and post-systolic regional contraction that together result in diminished stroke volume.
Cardiac resynchronization therapy (CRT), which is sometimes called biventricular pacing, is a new form of therapy for congestive heart failure caused by dilated cardiomyopathy. Several studies now document the remarkable benefits conferred by CRT on appropriately selected patients with heart failure.
CRT uses a specialized pacemaker to re-coordinate the contraction of the right and left ventricles in patients with heart failure.
In approximately 30% of patients with heart failure, an abnormality in the heart's electrical conducting system causes the two ventricles to beat in an asynchronous fashion. That is, instead of beating more or less simultaneously, the two ventricles beat slightly out of phase. This asynchrony greatly reduces the efficiency of the ventricles in patients with heart failure, whose hearts are already damaged.
CRT re-coordinates the beating of the two ventricles by pacing both ventricles simultaneously. This differs from typical pacemakers, which pace only the right ventricle.
Early studies with CRT demonstrated its ability to improve the symptoms, the exercise capacity, and the feeling of well-being of many patients with moderate to severe heart failure. Additional studies showed that CRT can improve both the anatomy and function of the heart—tending to reduce the size of the dilated left ventricle, and improving the energy usage of the heart.
Left ventricular dyssynchrony can lead to adverse cardiac effects, particularly if left untreated. As discussed above dyssynchrony can be improved by cardiac resynchronization therapy, but accurate diagnosis of the disorder is essential to obtain good results.
U.S. Pat. No. 7,082,329 relates to a method and monitor for monitoring diastolic relaxation of a heart ventricle by measuring an impedance signal in a ventricle of the heart and detecting the occurrence of a notch in the impedance signal coincident with the entry of blood into the monitored ventricle. The impedance signal is processed by studying the time derivative of the impedance signal which thus enables early detection of congestive heart failure.
In investigations it has been shown that cardiac mechanical dyssynchrony is visible in the morphology of the cardiogenic impedance (CI) signal. In addition it has been shown to study the fractionation of the CI-signal.
Fractionation means, in this context, that the curve length, e.g. being one or several additional notches in the CI-signal that may be identified if the signal is fractionated, or divided, in its parts. These notches, resulting in an increased curve length, are not present during synchronized contractions.
However, there exist no efficient way of automating the procedure of determining the fractionation, and in particular a procedure that requires a low level of processing capacity which is a presumption for use in an implantable heart stimulator having strict power consumption requirements.